Spark erosion cutting with numerical digital control of guidance of a wire electrode was originally introduced for making prismatic openings in workpieces. However, manufacturing problems frequently occur in making of cutting tools and in connection with sintered and extrusion dies where conically profiled blind surfaces or openings for stamping and punching are required in addition to a cylindrical opening. To avoid idle time and changing to other operating equipment it is necessary to seek a way in which this work can also be carried out on a single spark erosion cutting apparatus with only one work clamping operation. The angular slopes can be 45.degree. and more, so that the conventional wire guide means used on existing equipment cannot be directly employed, because when the wire electrode passes out of such a guidance means with such a large inclination, notch effects and increased abrasion on the wire result which can lead to inadmissible fluctuations in wire stress and to bending which cause permanent plastic deformation in the wire.
The presently known equipment for carrying out conical spark erosion cutting with a wire electrode preferably use small slope angles, of approximately less than 2.degree. and therefore only have control processes permitting the guidance of the displacement of the upper wire guidance device relative to the lower wire guidance device by small amounts V. There can be both a polar and a Cartesian guidance of the coordinates and the particular displacement amount V and its direction can either be set in a fixed manner (see German Pat. No. 2,155,622 and corresponding U.S. Pat. No. 3,830,996) or can be derived mathematically from digital control and adjusted continuously tangentially to control the instantaneous cutting direction (see Swiss Pat. No. 515,693 and corresponding U.S. Pat. No. 3,731,045 and Swiss Pat. No. 513,694 and corresponding U.S. Pat. No. 3,731,043). These publications describe accessories for mounting on existing machines, as well as devices which are integrally incorporated into the machines and in which the arms carrying the guidance heads are correspondingly moved (see German Patent disclosure document DE-OS No. 2,255,429). The guidance means of these devices cannot be oriented in relation to the slope of the wire and as a result large slope or inclination settings are not possible. It is known to use guidance means such as e.g. drawing dies or rollers in which the lateral removal angles can be larger than usual. However, in this case, the contact point of the trailing arc of the guidance means is displaced, so that on changing the conicity it is necessary to accept fluctuations regarding the machining geometry, and a degradation of precision. The resulting workpiece geometry errors may become so large that they cannot normally be accepted in tool construction. An example of this type is contained in Swiss Pat. No. 486,288 in which the signals for the conical regulating motors are obtained from the scanning of a template.
To obtain greater slope angles of the wire electrode, it has been proposed to fix the upper and lower wire guides to a common support and pivoting the latter by control devices (see German Patent Disclosure Document DE-OS No. 2,644,369). Quite apart from the difficult gimbal movement mounting of such a support and the limited range of adjusting the working height between the two guide heads, such a solution has the disadvantage that when inclined, the support may collide with the workpiece. It is not possible to adjust the height of the cone center position for a conical cut with respect to the workpiece. Another solution of this type is shown in East German Pat. No. 115,444.
A third construction type is known in which the workpieces are mounted on a gimbal suspension and are pivoted to bring about larger angles (see German Patent Disclosure Document DE-OS No. 2,401,424). However, since the workpieces generally have much larger dimensions than the almost dimensionless wire electrode, the design of a gimbal suspension for the suspension for the workpieces always involves a considerable machine construction expenditure. In addition, when a large workpiece is inclined, the pivoting movement of the outer workpiece portions can lead to collisions with the machine structure or the guidance arms, which must otherwise be set to particularly wide guidance distances. Thus, e.g. for an angular setting of 30.degree. at only 200 mm from the machining center, the pivoting stroke is already 100 mm (sine 30.degree.=0.5).